This invention relates to drive means in which cables or belts are used for transmitting drive force.
In motive force transmitting means that require a feed of high speed and high accuracy to be effected as in carriage drive means of a printer, for example, a drive force transmitting system using cables or belts and pulleys is widely used because it has the advantages of being simple in construction, low in inertial force and high in durability. The cable or belt used in this type of means is trained over pulleys with tension at all times to ensure that the carriate is driven and stops in positions which are the same throughout the operation. Moreover, during carriage drive operation, repeated stress is applied to the cables or belts and elongation thereof may be caused to occur as time passes. If this tendency is allowed to take place without any check, the cables or belts may be loosened and it will become impossible for the carriage to stop in predetermined positions without fail, causing a deterioration in the quality of the printed characters. Thus it is necessary to correct the elongation of the drive cables or belts that might occur with time.
FIG. 1 shows one type of carriage drive means of the prior art of a printer operative to tension the drive cable and provided with means for correcting elongation of the cable that might occur with time, in which a drive motor 1 has secured to its output shaft a drive pulley 2, a carriage 9 movable in a predetermined path along a guide, not shown, guide pulleys 5, 6, 7 and 8, and drive cables 3 and 4, the drive cable 3 being wound on the drive pulley 2 and connected to the carriage 8 while being trained over the guide pulleys 5 and 6 while the drive cable 4 is wound on the drive pulley 2 and connected to the carriage 9 while being provided over the guide pulleys 7 and 8, to provide a loop connecting the carriage 9 to the output shaft of the motor 1. The pulleys 7 and 8 are mounted on a tensioning arm 11 pivoted on a fixed pin 10 located near the pulley 7, and a tensioning spring 12 is mounted between one end of the tensioning arm 11 and a machine frame to urge the pulley 8 to move outwardly at all times. By this arrangement, the cables 3 and 4 are tensioned as predetermined at all times and elongation of the wires that might occur with time can be corrected.
The tensioning mechanism shown in FIG. 1 is not without disadvantages. If the tensioning spring 12 has a low tension, the vibration system consisting of the carriage 9, cables 3 and 4 and spring 12 would be low in natural frequency and large in amplitude with prolongation of vibration damping time. Thus it would take a long time before vibration is damped to an allowable amplitude level, adversely affecting the printing speed. If the printing speed is set at a high level, printing would be carried out before the amplitude is sufficiently damped, so that alignment of the printed characters would be unobtainable and the quality of the printed characters would be deteriorated.
On the other hand, if the tensioning spring has a high tension, the aforesaid disadvantages could be obviated. However, it would become necessary to increase the rigidity of the frame structure. This would not only increase costs but also shorten the lives of the pulleys, pulley bearings and cables. A friction load applied to the drive motor would be increased, and variations that occur with time would increase, thereby causing changes to occur in vibration characteristics. Moreover, when the tension of the spring is increased, the resilience of the frame with respect to the spring constant could not be neglected. Thus it is impossible to increase the spring constant to all intents and purposes.